A Micro Electro Mechanical System (MEMS) of mechanical elements combined with electronic circuit elements is provided with a hollow package to physically protect fine moving part from external environments. Such a package may be formed by a known method in which a sealing substrate provided with a sealing frame (sealing pattern) or a bump electrode having a height of some μm to dozens μm are formed by metal plating and bonded onto a MEMS substrate by thermocompression. The metal constituting the sealing frame or bump electrode is often made of gold (Au) which is excellent in workability as well as physical characteristics such as high electric conductivity, high deformability and high corrosion resistance.
Patent document 1 discloses package formation methods with their advantages and disadvantages, and discloses a method to bond them with a bonding material by thermocompression performed at a temperature lower than the melting temperature of the bonding material to soften without generating liquid phase. This method allegedly makes it possible to perform the thermocompression at a relatively low pressure while not requiring a process in which the bonded surface is highly smoothed (to lower roughness) or is deoxygenated by highly cleaned with extreme purity. The material for bonding the interface may be unalloyed metal such as Au, Sn, Cu and Al, a single phase of alloy such as AuSn, Au5Sn and AuIn, or mixture thereof.
Non-patent document 1 discloses a package formation method, in which a predetermined sealing frame is formed by photoresist development on titanium-sputtered thin film having thickness of 50 nm and gold-sputtered thin film having thickness of 200 nm formed on wafers made of silicon or glass and then a sealing material made of gold particles is provided on the sealing frame by using a screen mask to perform thermocompression between the wafers. The two wafers are heated to 300° C. in a vacuum chamber and then are bonded together at pushing pressure of 73 MPa for 30 min. The pushing pressure can be decreased by the sealing material made of gold particles, which is porous and easily deformed at the time of bonding.
Non-patent document 2 discloses a low-temperature bonding method, in which gold stud bumps are formed on a gold thin film of silicon substrate and are subject to a coining with silicon chip to make the gold bump have a smooth top face to be activated with nitrogen atmosphere plasma and then bonded onto a gold thin film electrode having a surface activated by the same method as the top face. The bumps are heated to 150° C. and bonded at pushing pressure of 320 MPa for about 30 sec. In the coining process, when the tip of bump is sharpened the smooth top face can be formed even at a low temperature and low pushing pressure.